This invention relates to fiber optic interferometric sensors that respond to perturbations such as acoustic wavefronts by producing a phase difference in two light beams propagated by fiber optic material. Still more particularly, this invention relates to methods and apparatus for cailbrating signal processing channels of fiber optic interferometric sensor arrays for use in sensing acoustic energy.
Optical fibers can be made sensitive to a large number of physical phenomena, such as acoustic waves and temperature fluctuations. An optical fiber to such phenomena changes the amplitude, phase or polarization of light guided by the fiber. Optical fibers have been considered for use as sensing elements in devices such as hydrophones, magnetometers, accelerometers and electric current sensors.
Mach-Zehnder, Michelson, Sagnac, and resonant ring interferometers have been used as sensors. Mach-Zehnder, Michelson and Sagnac interferometers respond to the phenomenon being sensed by producing phase differences in interfering light waves. Detecting phase changes in the waves permits quantitative measurements to be made on the physical quantity being monitored. The Sagnac interferometer produces phase differences in two counter-propagating light waves in a coil of a single fiber in response to rotations about the axis of the coil.
The Mach-Zehnder interferometer is particularly suited for sensing acoustic vibrations. A fiber optic Mach-Zehnder interferometer typically has a reference arm comprising a first length of optical fiber and a sensing arm comprising a second length of optical fiber. An optical coupler divides a light singal between the two arms. The sensing arm is exposed to a physical parameter, such as an acoustic wavefront, to be measured while the reference arm is isolated from changes in the parameter. A second coupler recombines the signals after the sensing arm is exposed to the parameter to be measured. When the Mach-Zehnder interferometer is used as an acoustic sensor, acoustic wavefronts change the optical length of the sensing arm as a function of the acoustic wave amplitude. The signals are recombined after they have propagated through the reference and sensing arms, and the phase difference of the signals is monitored. Since the signals in the reference and sensing arms had a definite phase relation when they were introduced into the arms, changes in the phase difference are indicative of changes in the physical parameter to which the sensing arm was exposed.
The Michelson interferometer is similar to the Mach-Zehnder except that the Michelson interferometer includes only one coupler. Mirrors terminate the ends of the fibers and reflect the light back toward the input.